1. Field of the Invention
The present invention relates to improvements in a power semiconductor switching device of gate turnoff type that can be turned on or off from its control electrode.
2. Description of the Prior Art
Gate turnoff thyristors, static induction transistors, or the like, each of which will be referred to as a GTO, have been known as power semiconductor switching elements suitable for the interruption of large electric currents. Referring now to FIG. 10, there is illustrated a schematic circuit diagram of a prior art power semiconductor switching device including a power semiconductor switching element such as a GTO. In the figure, reference numeral 1 denotes a semiconductor element, such as a GTO, having its anode electrode A, its cathode electrode K, and its gate electrode (i.e., control electrode) G, 20 denotes a reverse bias driving circuit for applying a reverse bias between the gate and cathode electrodes G and K, 2 denotes a storage element, such as a capacitor, for storing an electric energy to apply a reverse bias between the gate electrode G and the cathode electrode K of the semiconductor element 1 such as a GTO, and 3 denotes a reverse bias switch which can be brought into conduction so as to cause the semiconductor element 1 to make a transition from its on state to its off state by applying the electric energy stored on the storage element 2 between the cathode and gate electrodes K and G backwardly.
Reference string Son denotes a forward-bias signal terminal to which a forward bias signal is applied, Soff denotes a reverse-bias signal terminal to which a reverse bias signal is applied, Vs denotes a reverse-bias power supply terminal, B denotes a reverse-bias power supply, and COM denotes a common terminal which defines the potential of the cathode of the semiconductor element 1.
In operation, the reverse bias power supply B stores an electric energy in the storage element 2 for applying a reverse bias between the gate and cathode electrodes G and K of the semiconductor element 1. The electric energy cannot be used to apply a reverse bias between the gate and cathode electrodes G and K of the semiconductor element 1 such as a GTO so long as the reverse bias switch 3 is kept in its nonconductive state. When the semiconductor element 1, such as a GTO, remains in its off state first, the application of a forward bias signal via the forward bias signal terminal Son with respect to the common terminal COM causes the semiconductor element 1, such as a GTO, to make a transition from its off state to its on state.
In order to cause the semiconductor element 1, such as a GTO, which has been turned on to make a transition to its off state, it is necessary to set and hold the anode current at zero for a predetermined period of time without the application of the forward bias signal to the forward bias signal terminal Son, or to apply a reverse bias between the gate and cathode electrodes G and K of the element 1. In the latter case, the reverse bias switch 3 can be brought into conduction by applying a reverse bias signal from the reverse bias signal terminal Soff to the reverse bias switch 3 so that the electric energy stored in the storage element 2 is used for applying a reverse bias between the gate and cathode electrodes G and K of the semiconductor element 1.
In such the prior art power semiconductor switching device, a connection between the driving circuit 20 for controlling the conduction between the anode and cathode electrodes A and K and the semiconductor element 1 such as a GTO is established by lead lines extended from the cathode and gate electrodes K and G. While it is necessary to precisely control the conductivity of the power semiconductor switching device by means of the reverse bias driving circuit 20 in order to improve its responsivity and reliability, it is difficult to generate a sufficiently large reverse bias gate current having a sufficiently large varying rate of (-dIg/dt), where Ig is the reverse bias gate current, because of the impedance of the lead lines which poses a barrier to the generation of such a rapidly-varying large reverse bias gate current. This results in restricting the interruption capability of the semiconductor element 1 such as a GTO. It is thus difficult to improve the interruption capability of the semiconductor element 1 within the prior art power semiconductor switching device.
Europe Patent No. 0 328 778 B1 discloses a concept of packaging a power semiconductor device. Referring next to FIG. 11, there is illustrated a diagram showing the structure of a prior art power semiconductor device in which the packaging concept is made concrete. In the figure, reference character W denotes a power semiconductor wafer, Aa denotes a first copper block electrode which serves as an anode electrode, Ka denotes a second copper block electrode which serves as a cathode electrode, Kb and KC denote extended cathode electrodes which are electrically connected to the second copper block electrode Ka, and Ga denotes a cylindrical-shaped gate electrode which is brought into contact with the gate surface of the power semiconductor wafer W.
In the packaged power semiconductor device shown in FIG. 11, the reverse bias driving circuit is constructed of a number of series circuits each comprised of a capacitor 2a and a reverse bias switch 3a. The number of series circuits are arranged between the cylindrical-shaped gate and expanded cathode electrodes Ga and Kc. The reverse bias driving circuit constructed of the number of series circuits each comprised of one capacitor 2a and one reverse bias switch 3a is thus housed together with the power semiconductor wafer W within the same package.
Although such the packaging concept for power semiconductor switching devices offers the hope of improving its interruption capability since the inductance caused by the lead line extended from the gate electrode in the prior art switching device shown in FIG. 10 is eliminated and hence the varying rate (-dIg/dt) of the reverse bias gate current is increased, it suffers from a disadvantage that the voltage drop in each reverse bias switch 3a such as a MOSFET is increased and hence its conduction capability is reduced because all the components of the reverse bias driving circuit are disposed within the package including the power semiconductor wafer W held at a high temperature of 125.degree. C.
Another problem with the packaged power semiconductor switching device is that each capacitor 2a cannot withstand exposure to such the high temperature, or reduction in the capacitance caused by size constraints makes it difficult to feed a sufficient reverse bias current, or supply a sufficient charge which is the time integral of a sweep out current. For example, electrolytic capacitors and organic semiconductor capacitors cannot be used because of their low heat resistance from the viewpoint of their needed useful life. Laminated ceramic capacitors cannot be used because of their small capacitances and because their capacitances are decreased remarkably when they are placed in environments at high temperatures as mentioned above. If such capacitors are forcedly incorporated into the prior art power semiconductor switching device as shown in FIG. 11, the reliability of the components which construct the reverse bias driving circuit is reduced.